A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The known lithographic apparatus comprises a position control system or controller to control the position of a substrate support supporting the substrate. This position control system comprises a position measurement system which is configured to measure a number of sensor or sensor target positions of the substrate support.
During use of the lithographic apparatus, forces will be exerted on the substrate support. For instance, during the expose phase, i.e. during projection of the patterned beam on a target portion of the substrate level, level actuations will be performed to position the upper surface of the substrate in a correct orientation with respect to the lens column. As the stiffness of the substrate support is limited, the level actuations may cause temporary deformations of the substrate support. Also, thermal conditions may result in deformations in the substrate support. These deformations may lead to focus errors and/or errors in overlay and CD uniformity
To reduce the risk of deformations of the substrate table and as a consequence focus errors or overlay offset, it has been proposed to increase the stiffness of the substrate support by providing a stiffer structure. However, due to the increasing demand on accuracy and speed of the positioning of the substrate support, the possibilities to increase the structural stiffness of the substrate support structure without encountering further problems, for instance with respect to weight have come to their limits.
These limitations in flexibility of the substrate support as a result of increasing accelerations may also be encountered in the position control of other objects such as the patterning device support.
US 2011/0026004, the contents of which are herein incorporated by reference, discloses a lithographic apparatus comprising a stiffening system to increase the stiffness and/or to damp relative movements within the body of an object, the stiffening system comprising;
one or more sensors, wherein each sensor is arranged to determine a measurement signal representative for an internal strain or relative displacement in said body,
one or more actuators, wherein each actuator is arranged to exert an actuation force on a part of said body, and
at least one controller, configured to provide on the basis of said measurement signal of at least one of said sensors, an actuation signal to at least one of said actuators to control movements within said body, in particular to increase the stiffness and/or to damp movements within said body.
The sensors proposed in US 2011/0026004 are optical measurement sensors using laser interferometry, or, as an alternative, high performance strain gauges. By sensing of the shape of the object using laser interferometry, dynamic deformations within the object can be measured and counteracted resulting in a more stable shape of the object.
A drawback of the use of multiple interferometers in the object is that the resulting measurement system using laser interferometry is relative expensive and may be difficult to incorporate due to volume conflicts with other components.